Identification system for monitoring the presence/absence of members of a defined set

ABSTRACT

A system which allows multiple senders to asynchronously transmit identification codes via a common communication channel (e.g., RF) to enable a central monitor to identify the presence (or absence) of each sender within the monitor&#39;s detection zone. Each sender is configured to repeatedly transmit a uniquely encoded identification frame. A frame, in accordance with the invention, is comprised of pulses spaced to minimize pulse collisions and configured to tolerate occasional collisions without impairing the monitor&#39;s ability to separately identify each transmitting sender. Each sender is configured to repeatedly transmit a unique identification frame characterized by a pulse pattern comprised of active intervals spaced by inactive (or “quiet”) intervals. The inactive intervals have variable length durations which are preferably pseudorandomly selected so that each sender defines a unique sequence of inactive interval durations.

This application is a national stage filing under internationalapplication PCT/US00/13478 pursuant to 35 U.S.C. 371 which claim benefitof 60/135,452 file May. 21, 1999.

FIELD OF THE INVENTION

This invention relates generally to systems e.g., radio frequencyidentification (RFID), for identifying the presence or absence ofmembers of a defined member set. More particularly, the invention isdirected to a system which includes multiple active senders and acentral monitor, with each sender being configured to repeatedlytransmit a signal representing a uniquely encoded identification frame.A composite signal received by the monitor is decoded to separatelyidentify each of the transmitting senders.

BACKGROUND OF THE INVENTION

Various RFID systems are described in the patent and technicalliterature which are used to identify the presence of speciallyconfigured identification tags within a prescribed zone. Depending uponthe system application, the tags can be affixed to a set of inanimateobjects, or animals, or humans. Prior literature describes both passivetags which require interrogation and active tags which are powered andcan initiate an identification signal transmission.

SUMMARY OF THE INVENTION

The present invention is directed to a system which allows multiplesenders to asynchronously transmit identification codes via a commoncommunication channel (e.g., RF) to enable a central monitor to identifythe presence (or absence) of each sender within the monitor's detectionzone. Each sender is configured to repeatedly transmit a uniquelyencoded identification frame. A frame, in accordance with the invention,is comprised of pulses spaced to minimize pulse collisions andconfigured to tolerate occasional collisions without impairing themonitor's ability to separately identify each transmitting sender.

Systems in accordance with the invention are useful in a wide variety ofapplications for monitoring the presence (or absence) of a member of adefined set of members. For example, the teachings of the invention canbe used in a warehouse system to log the presence of items in inventoryor in a man-overboard (MOB) system for monitoring the absence of personson a recreational boat. The indication of presence or absence can beused to either directly actuate an output device, e.g., an alarm, or canbe logged for further processing by a host computer. In such systems,each member of the set (e.g., each inventory item or each person on theboat) carries a sender.

Senders, in accordance with preferred embodiments of the invention,comprise small inexpensive electronic devices which can be carried by,attached to, or worn by the members of the set being monitored. Eachsender is preferably self powered, typically by a battery, and iscapable of periodically transmitting, low power, short duration pulsesover a long life which can, in some applications, be measured in years.A power level is selected to support a communication range appropriateto the application; e.g., from less than one hundred to several hundredfeet.

In accordance with the invention, each sender is configured torepeatedly transmit a unique identification frame characterized by apulse pattern comprised of active intervals spaced by inactive (or“quiet”) intervals. The inactive intervals have variable lengthdurations which are preferably pseudorandomly selected so that eachsender defines a unique sequence of inactive interval durations.

Active intervals in accordance with the invention are comprised of oneor more pulse intervals. That is, in a first embodiment, each activeinterval can comprise a coded burst of multiple pulses. In a differentembodiment, each active interval comprises a single pulse. A quietinterval is defined between successive active intervals.

In accordance with certain preferred embodiments of the invention, thedurations of quiet intervals within a frame are used to define theidentification code (i.e., ID code) of each sender. As an example, anidentification frame format can comprise six active intervals boundingfive quiet intervals. The duration of each quiet interval can, forexample, have a resolution of between 1 and 64 clock periods. Theseexemplary quantities would provide for a pool of 64⁵ possible ID codes.A different ID code is assigned to each of the multiple senders. Atypical clock period can equal 100 microseconds, for example, so that anidentification frame using the aforementioned exemplary numbers couldhave a duration of up to 32.6 milliseconds [i.e., (6 activeintervals*100 microseconds)+(5 quiet intervals * 64*100 microseconds)].In such an exemplary embodiment, the pulse duty cycle of each sender isextremely low, i.e., the sender transmits for only 600 microseconds outof each identification frame duration of 32.6 milliseconds. In certainapplications of the invention, sender transmission cycles can becomprised of an identification frame and a silent period. This furtherreduces the duty cycle and power consumption of each sender.

Moreover, the number of active senders sharing a common channel in atypical application of the invention is several orders of magnitude lessthen the number of possible ID codes. All of the foregoing factorscontribute to minimize the occurrence of pulse collisions on the channeland enable a central monitor to interpret a received composite signal toreliably and unambiguously identify each transmitting sender.

More particularly, embodiments of the invention are characterized bysender pulse patterns which are inherently sparse, i.e., low density,and inherently redundant. Accordingly, embodiments of the invention areable to tolerate pulse collisions by effectively treating collidingpulses as belonging to every sender that might otherwise be identifiedby other pulses in the received composite signal. Because of the verylow average transmission time of each sender and the inherent toleranceto pulse collisions, embodiments of the invention are able toaccommodate a large number of asynchronously transmitting senders.

A central monitor in accordance with the invention includes a memory forstoring a repertoire of sender ID codes. This repertoire can be storedor acquired in several different ways; e.g., the ID codes can be 1)preprogrammed into nonvolatile memory at the factory, 2) uploaded from ahost processor, 3) programmed into monitor memory via a manual agentsuch as a keypad or switches, 4) learned by the monitor during aninstallation or set-up procedure, 5) learned by direct electricalcontact or by an auxiliary channel such as an infrared (IR) lightemitting diode (LED), 6) learned by repeated recognition. Regardless ofhow the ID code repertoire is acquired, the monitor operates to compareeach ID code stored in its repertoire against a received compositesignal history accumulated by the monitor. More particularly, themonitor includes a memory which preferably accumulates a history of thecomposite signal pulse pattern over a time equivalent of at least thelongest ID frame. The history is updated at a rate to reflect thereceived pulses and intermediate interval durations. Each time a newpulse is received, the monitor is able to compare the stored ID codeswith the history to identify a match.

In certain applications of the invention, e.g., a man-overboard system,each sender is expected to “check-in” within a certain “time-out”period,e.g., every three seconds. If a sender's ID code fails to appear in thereceived composite signal within the time-out period, then the monitorgenerates an alarm. In systems intended for other applications, themonitor can respond more quickly or more slowly or can merely maintain alog of the presence or absence of a sender. In still other systems, themonitor operates to report sender check-ins to a host computer.

In accordance with a first exemplary embodiment (serial code burst) ofthe invention, each sender transmits a coded burst of pulses during eachframe active interval. Each pulse burst defines an ID code uniquelyidentifying the sender. The bursts are spaced by quiet intervals whosedurations are preferably pseudorandomly selected. Thus each sender willtransmit its bursts based on a different sequence of quiet intervaldurations so that successive burst collisions are highly improbable.

In accordance with a second exemplary embodiment (code burst/pulse) ofthe invention, a coded pulse burst defining a sender's ID code istransmitted once per frame and marker pulses are additionallytransmitted during each frame to bound the quiet intervals. In thisembodiment, the sequence of quiet interval durations can redundantlyidentify the transmitting sender.

In a third exemplary embodiment (pulse only) of the invention, the IDcode of each sender is defined by a unique sequence of quiet intervaldurations between marker pulses transmitted during each frame. Themonitor includes a storage means storing a repertoire of sender IDcodes, i.e., a set of interval durations for each sender. Additionally,the monitor includes a memory, e.g., a shift register, which maintains ahistory of recently received composite signal pulses. Each time a newpulse is received, the monitor compares the pulse pattern of each storedsender ID code against the pulse history looking for a match. When amatch is recognized, the associated sender is confirmed as having“checked-in” and a free running timer associated with that sender isreset. As long as the timer is reset before it counts to a thresholdtimeout value, the sender is deemed to be continuously present.

In a fourth exemplary embodiment (pulse/group synch) of the invention,the pulse pattern of the third embodiment is supplemented by a groupsynch (GS) pulse pattern common to all of the senders. Thus, the GSpattern defines the set of members and enables monitor to more rapidlyidentify the sender ID codes within the composite signal received fromall senders.

In a fifth exemplary embodiment (group synch/symbol) of the invention,the fourth embodiment ID code pulse pattern is subdivided into multiplesub-codes. In this embodiment, each sub-code is able to communicate adifferent symbol to the monitor. Symbols, for example, might be numbers.That is, each symbol could be used to represent a number from 0 to 99.With an ID pattern defining three such symbols, the monitor can discern1,000,000 possible unique sender-identifying ID codes. This embodimentfinds service in applications where the population of the member set islarge but relatively few members are present at any given time. Anexample of such an application is the monitoring and logging ofwarehouse items flowing down a conveyor belt for shipment.

Several variants of the aforementioned embodiments are also describedhereinafter. For example, a correlation processing capability can beadvantageously introduced into the several embodiments. This capabilityinvolves the process of correlating a received data stream with pulsepatterns associated with known sender ID codes. The known sender IDcodes are stored as binary patterns in the monitor's repertoire memory.The received data stream originates as a graded signal, e.g., a radiobaseband signal. This graded signal can be converted to digital formatand processed arithmetically or the received data stream can be firstprocessed by a thresholding element so that the data is converted to abinary pulse train format. In either case, correlation processinginvolves comparing two signals and computing a correlation score. Theresulting scores are compared to discriminate threshold in order to makea decision as to the presence or absence of a particular sender.Correlation processing permits enhanced detection of sender pulsepatterns when the incoming data is partially corrupted or incomplete.

In another variant, one or more data pulses can be positioned in an IDframe to enable a sender to communicate data, e.g., temperature or flowrate, to the monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical system in accordance with theinvention including multiple senders A-H and a central monitor;

FIG. 2A depicts a typical sender in accordance with the inventionintended to be carried by a user via a neck-strap;

FIG. 2B is a functional block diagram of a typical sender in accordancewith the present invention;

FIG. 3 is a timing diagram in accordance with the first (serial codeburst) embodiment of the invention showing within each frame multiplecode bursts and a sequence of quiet intervals;

FIG. 4 is a block diagram depicting a monitor useful with the firstembodiment of FIG. 3 for decoding and responding to transmissions frommultiple senders;

FIG. 5 is a flow chart depicting an operational sequence for the monitorof FIG. 4;

FIG. 6 is a timing diagram in accordance with the second (codeburst/pulse) embodiment of the invention showing within each frame asingle code burst and multiple marker pulses to define a sequence ofquiet intervals;

FIG. 7 is a block diagram depicting a monitor useful with the secondembodiment of FIG. 6 for decoding and responding to transmissions frommultiple senders;

FIG. 8 is a flow chart depicting an operational sequence for the monitorof FIG. 7;

FIG. 9 is a timing diagram in accordance with the third (pulse only)embodiment of the invention showing a frame format including multiplemarker pulses defining a sequence of quiet intervals;

FIG. 10 is a block diagram depicting a monitor useful with the thirdembodiment of FIG. 9 for decoding and responding to transmissions frommultiple senders;

FIG. 11 is a flow chart depicting an operational sequence for themonitor of FIG. 10;

FIG. 12 is a timing diagram in accordance with the fourth (pulse/groupsynch) embodiment of the invention showing within each frame the markerpulses exemplary of FIG. 9 supplemented by a group synch pulse pattern;

FIG. 13 is a block diagram depicting a monitor useful with the frameformat of FIG. 12 for decoding and responding to transmissions frommultiple senders;

FIG. 14 is a flow chart depicting an operational sequence for themonitor of FIG. 13;

FIGS. 15 and 16 are block diagrams similar to FIG. 13 but modified toinclude a correlation detection capability;

FIG. 17 is a block diagram similar to FIG. 13 but modified to introducea symbol based detection capability;

FIGS. 18, 19, and 20 represent frame format variants depicting theinclusion of one or more data pulses within a frame;

FIGS. 21 and 22 are block diagrams similar to FIG. 9 but expanded toexemplify how the monitor repertoire can acquire ID codes;

FIGS. 23A, 23B, 23C1, 23C2, 23C3, 23C4, 23D and 23E schematically depictarrangements for conserving sender battery power;

FIGS. 24A and 24B schematically depict arrangements for discerningsender battery status; and

FIG. 25 is a functional block diagram of a system in accordance with theinvention using a wire as the communication medium.

DETAILED DESCRIPTION SENDER/MONITOR SYSTEM (FIG.1)

Attention is initially directed to FIG. 1 which depicts a typical system20 in accordance with the invention comprised of multiple senders 18(respectively labeled, A-H) and a common monitor 22. The system 20 canbe used in a wide variety of applications to keep track of the membersof a predetermined set of members. For example, the set of members canbe all persons on board a boat. Each member carries a different sender,e.g., on a neck-strap, as depicted in FIG. 2A. The function of eachsender is to repeatedly transmit an identification (ID) code whichuniquely identifies the sender. An ID code in accordance with theinvention is preferably represented by a pulse pattern contained in anidentification frame transmitted during each sender cycle.

Systems in accordance with the invention can accommodate widelydifferent numbers of senders depending on the application. For example,in a typical man-overboard system intended for use with recreationalboats, it may be sufficient for the system to accommodate a small numberof senders, e.g., twelve. In a system intended to monitor animals toprevent their wandering out of a defined area, it may be appropriate toaccommodate one hundred or more senders. In other applications, forexample, to monitor children at a school, or elderly persons at a seniorhome, or inmates at a prison, it may be appropriate to accommodateseveral hundred senders. In other applications, for example, a railfreight yard, a system in accordance with the invention can operate inconjunction with a host computer to monitor and log individual box carsas they enter a yard. The number of box cars actually logged may belimited but the population of different box cars capable of beingdistinguished and individually logged could be very large, e.g., manythousands.

FIG. 2A depicts a typical sender 24 comprising a housing 26 having aflexible antenna 28 and carried by a neck-strap 30 intended to be wornby a member to be monitored. FIG. 2B depicts a functional block diagramof electronic circuitry within housing 26 and connected to antenna 28.The circuitry includes a battery 34 for powering a controller 36 and atransmitter 38. The controller 36 functions to amplitude control (e.g.,on-off) a carrier signal generated by transmitter 38 to produce a pulsepattern defined by a unique ID code stored in the controller. Althoughit is primarily contemplated that the carrier signal comprise an RFsignal, it is important to recognize that the invention can also beimplemented using other communication mediums such as a common wireconnecting all senders to the monitor, ultrasound, infrared, etc. It isalso pointed out that the controller 36 can be implemented in variousways such as a programmed microprocessor, state machine, hardwiredlogic, etc.

CODE BURST EMBODIMENT (FIG. 3)

Attention is now directed to FIG. 3 which depicts an exemplaryidentification frame transmitted by a sender 24 in accordance with thefirst (serial code burst) embodiment. In this embodiment, anidentification frame is depicted as being comprised of active intervalsB1, B2, B3, B4, B5, each containing a pulse burst 50, spaced by quietintervals Q1, Q2, Q3, Q4, Q5.

In the embodiment of FIG. 3, each sender transmits identical coded pulsebursts 50 of On/Off-Keyed pulses which define a binary ID code. A burstcan, for example, consist of ten pulse intervals which would yield apool of 1024 unique ID codes. The duration of each pulse burst 50 ispreferably short relative to the duration of a quiet interval 56 betweenbursts. Thus, quiet intervals 56 in FIG. 3 preferably have a durationconsiderably greater than ten pulse intervals.

More particularly, each code burst 50 in FIG. 3 is comprised of a seriesof pulse intervals, each pulse interval accommodating a single code bit.The presence or absence of a transmitted signal during a pulse intervaldetermines the binary state of the code bit. The ID code developsserially in time so it is vulnerable to interfering signals from othersenders throughout the burst 50. The embodiment of FIG. 3 is configuredto reduce the prospect of such interference (collisions) among senders'signals and to mitigate their effect when they rarely occur.

In the embodiment of FIG. 3, successive code bursts from a sender arespaced by irregular but preset quiet intervals depicted as Q1, Q2, Q3,Q4, and Q5. In practice, the quiet interval duration is much longer thanthe active interval duration. The duration of each quiet interval isinternally programmed at the sender's controller 36, as is the sender'sID code manifested in burst 50. The respective durations of the quietintervals are preferably pseudorandomly determined. Each sender has aunique serial ID code assigned to it as well as a unique set of quietinterval durations. The identification frame duration need not be thesame at each sender but it is important that no two senders have thesame sequence of quiet interval durations or the same ID code.

In the embodiment of FIG. 3, when multiple senders are participating, afirst collision will occur with a mathematically predictable probabilityat a rate dependent on system pulse density which is effected primarilyby the number of senders present, the duration of the sender's codeburst, and the range of quiet interval durations between bursts. Theprobable incidence of collision can be readily calculated but, ingeneral, fewer senders and shorter code bursts result in fewercollisions. Note, however, that the probability of sequential collisionsbetween any particular pair of sender signals is very much reduced sincethe quiet interval following the first collision is different for eachof the collision participants.

In a practical system for absence detection, a low probability of false“alarm” is obtained by programming the monitor 22 to allow multiplecycles of missing signal check-in before making a decision that a senderis absent. While a pulse collision could indeed occur during one cycleresulting in a missed sender check-in, the probability of a givensender's signal participating in a second collision in the succeedingcycle is much reduced. Likewise, it is even more reduced in a thirdcycle, etc.

FIG. 4 depicts an exemplary monitor 60 for use with the serial codeburst embodiment of FIG. 3. The monitor 60 includes a controller 62which includes a “code match detection” module 64. Code bursts arereceived from antenna 67 by receiver 68, which will be assumed toinclude a thresholding function so that it delivers a binary output.Module 64 functions to locate the matching sender code pattern A-H fromits repertoire stored in file 69 to identify the transmitting sender.The match signal outputs from module 94 are, for example, supplied to ahost computer and/or used to reset the appropriate free running timerA-H contained in timer module 70. The timer outputs from module 70 arecoupled through OR gate 71 to an audible signal 72 which is actuatedwhenever one of the timers 70 times out before being reset. The time outcan alternatively actuate a visible display or provide data to a hostcomputer.

FIG. 5 depicts the operational flow of the monitor 60 enabling it toprocess the sender signals and control the timers of module 70. Moreparticularly, in block 74, timer module 70 selects a next timer A-H andincrements its time count. Decision block 75 determines whether theselected a timer has timed-out. If yes, then an alarm (either audiblealarm 72 or LED alarm 73) is initiated. If no, operation proceeds todecision block 76 to determine whether a new sender code burst hasarrived. If yes, module 64 searches repertoire file 69 (block 77) andthe corresponding module 70 timer is reset in block 78. Operation thenloops back to block 74.

CODE BURST/PULSE EMBODIMENT (FIG. 6)

Attention is now directed to FIG. 6 which depicts the second (codeburst/pulse) embodiment. In this embodiment, the frame is comprised ofactive intervals, e.g., B1, B2, B3, B4, B5, B6 and quiet intervals Q1,Q2, Q3, Q4, Q5. One active interval, e.g., B1, accommodates a pulseburst 80 defining an ID code. Each of the other active intervals, i.e.,B2-B6 accommodates a single pulse which functions as a marker to bound aquiet interval. The quiet interval durations are preferablypseudorandomly selected and each, for example, has a width between 1 and64 pulse widths. Each sender has a unique ID code represented by burst80 and a unique sequence of quiet interval durations Q1-Q5.

FIG. 7 depicts monitor 90 for use with senders transmitting frames usingthe code burst/pulse format shown in FIG. 6. Monitor 90 includes a matchdetection module 94 which stores a repertoire of ID codes for sendersactive in the system and compares each code burst 90 from receiver 95with the repertoire of stored ID codes. When a stored ID code matches areceived code burst, module 94 produces a code match signal on anassociated output line 96. The code match signal performs twosignificant functions; (1) it resets an associated timeout timer inmodule 97 and (2) it accesses an associated pulse position data module98. Module 98 contains a separate register for each sender whichrepresents of the quiet interval durations unique to that sender. Theduration of a quiet interval, e.g., Q3, indicates the position (or time)that the next pulse, e.g., B5, should be received from receiver 95.

More particularly, when a code match signal occurs on an output line 96,it functions to transfer a set of pulse times (or quiet intervaldurations) into output register 99 along with information identifyingthe associated sender. These pulse times are then used to gate theoutput of receiver 95 to reset the associated timer in module 97 if apulse is received at the appropriate time. Note that the quiet intervaldurations Q1, Q2, . . . are transferred from register 99 into revolvingshift register 100 which delivers a “pulse gate” signal on line 101coincident with the expected pulses at times B3, B1, . . . . The datatransferred from register 99 includes sender identification data as wellas gate time data.

If receiver 95 provides a pulse on line 102 coincident with the gatesignal, the match will be recognized by pulse/gate match detectionmodule 103 which will produce a pulse match signal on an associatedoutput line 104. The pulse match signal functions to reset theassociated timer in module 97. Each timer module 97 is free running andwill time out unless reset by a code match signal on line 96 or a pulsematch signal on line 104.

If any of the timers in module 97 time out, the associated sender willbe deemed absent and the alarm display 105 will be signaled.

The monitor 90 functions not only to define pulse gate signals and tolook for a coincident pulse from receiver 95, but it also functions toproduce a code gate signal on line 106 to look for a sender code burst.The code gate signal 106 is produced by the revolving shift register 100in accordance with the gate times read out of register 99. Shiftregister 100 also provides sender selection data on line 107 which isused by the code/repertoire match detection module 95 and by thepulse/gate match detection module 103.

FIG. 8 is a flow chart depicting the operations sequentially executed bymonitor 90 in FIG. 7 with respect to a single sender. It should beunderstood that the same operational sequence shown in FIG. 8 isexecuted for each sender with the operations being interleaved. FIG. 8assumes that the monitor 90 has already acquired a particular sender,i.e., recognized its ID code as corresponding to a code stored in itsrepertoire file in module 94.

Block 110 in FIG. 8 is executed when the code gate signal is produced online 106 indicating the expectation of a sender serial code burst.Decision block 112 asks if the serial code is present. If yes, block 114is executed to reset the associated timer in module 97. If the output ofdecision block 112 is negative or after the execution of block 114,operation proceeds to block 116 which reads the associated pulseposition data file from module 98. As previously described, thisinvolves transferring the pulse position data and sender identificationto the revolving shift register 100, via output register 99. The shiftregister then sequentially produces gate signals on pulse gate line 101coincident with the expected arrival of pulses represented at B2, B3 . .. in FIG. 6. Decision block 118 then ask whether pulse n (i.e., B2) ispresent on receiver line 102. If yes, the associated timer in module 97is reset in block 120. If no, or after block 120, shift register 100 isread in block 122 to determine the position of the next expected pulse.Decision block 124 asks if pulse n−1is (i.e., B3) present. If yes, theassociated timer module 97 is reset in block 126. If no, or after theexecution of block 126, the next pulse gate signal is developed in block128. Again decision block 130 asks if receiver 102 provides a pulsecoincident with the next pulse gate signal.

Thus as is represented in FIG. 8, the monitor 90 determines at eachactive interval B1, B2, B3, B4, B5, and B6 of a frame whether or not thesender is checking in. If a sender fails to check in by providing a codeburst or pulse when expected, then its associated timer will continue torun. When the timer times out, the alarm display 105 is actuated.Depending upon the application, the system can be configured todisregard a limited number of successive check in failures. For example,in a critical man overboard application, the system might ignore failedcheck-ins up to a short interval of perhaps three seconds, whereas inother less critical applications this window could be considerablyextended.

PULSE ONLY EMBODIMENT (FIG. 9)

Attention is now directed to FIG. 9 which depicts the frame format of athird (pulse only) embodiment. In contrast to the formats of FIGS. 3 and6, the format of FIG. 9 does not use a coded pulse burst to define an IDcode. Rather, a sender's ID code is fully defined by the relativepositioning of multiple ID pulses, respectively designated as P1, P2,P3, P4, P5, which mark the boundaries quiet intervals Q1, Q2, Q3, Q4.The ID code can be properly viewed as being fully defined by thesequence of the quiet interval durations or, in other words, by thepositions (or time slots) of the ID pulses. The quantitative aspect ofthe FIG. 9 format can vary considerably based upon the intended systemapplication. However, for clarity, the embodiment of FIG. 9 will bedescribed with reference to certain exemplary parameters including (1)active intervals per frame equal five, each active interval comprising asingle pulse interval respectively identified as P1-P5, (2) pulseinterval equals 100 microseconds, (3) quiet intervals per frame equalfour, each quiet interval having a variable duration between 1 and 64pulse intervals, i.e., between 100 microseconds and 6.4 milliseconds.

Each sender has a unique sequence of quiet interval durations assignedto it to define its unique ID code. The durations are preferablypseudorandomly determined. With four quiet intervals per ID frame andwith each quiet interval having up to 64 different values, a very largenumber (64⁴) of different ID codes is available for assignment. Inasmuchas this number is several orders of magnitude greater than the number oftypically active senders, judicious code assignments would enable themonitor (FIG. 10) to identify an ID code by examining only a few of thepulse positions P1-P5. However, as will be described in connection withFIG. 10, by examining more pulses than is minimally required todistinguish a sender, redundancy is achieved in the code itself. Thispulse redundancy allows senders to be readily distinguished even in anenvironment of considerable interference from other senders and noisesources.

FIG. 10 depicts a monitor 200 for receiving and interpreting a compositesignal formed by multiple senders asynchronously transmitting inaccordance with the pulse only frame format of FIG. 9. Recall that eachsender repeatedly transmits its uniquely coded ID frame with a regularperiodicity. FIG. 10 shows a receive antenna 202, a thresholded receiver204 providing a binary output, a controller 206, and an alarm device208. The controller functionally defines a long shift register 212 forstoring a history of received pulses. The output of receiver 204 islogged into shift register 212 by clock ticks from clock 214. Forpurpose of explanation, assume that the clock 214 generates a clock tickevery 100 microseconds. Each clock tick will enter either a “1” or “0”bit into the shift register 212 depending upon the state of thecomposite output signal supplied by receiver 204. The shift register 212is selected to be sufficiently long to maintain a pulse history equal toat least the duration of the longest ID frame. The controller 206functions to compare its stored repertoire of ID codes (i.e., quietinterval durations/pulse positions) against the pulse historyrepresented by the shift register 212. If a match occurs, it means thatthe associated sender has checked in and consequently the associatedtimeout timer has been reset. If a timer's time out period expiresbecause its sender has failed to check in, then the alarm andannunciator 208 is alerted to the change of status.

More particularly, note that the controller 206 includes a sequencecontrol module 220. The sequence module 220 receives a timing signal vialine 222 whenever a pulse is entered into the shift register 212. Thesequence control module 220 via line 221 sequentially sends ID file froma repertoire stored in repertoire file storage 224. This causes thestorage 224 to transfer the selected set of quiet ID interval durations(or pulse positions) to file 226. File 226 stores the pulse times orpositions defined by the sequential quiet interval ID. The sequencecontrol module 220, via control line 230, sequentially transfers thequiet interval durations to register 234. The interval duration value inregister 234,via address selector 236, addresses a particular bitposition in shift register 212 to determine whether it stores a pulse.Shift register readout line 240 indicates whether a match is found. Ifsuccessive pulse matches are found, as described in FIG. 11, then thecontrol module 220 generates a reset signal to reset the associatedsender timer in timer module 242. If ever a sender fails to check inbefore the expiration of the timer's timeout period, then the alarm 208is actuated. Alternatively, or additionally, the reset signal can besupplied to a host computer to indicate presence.

FIG. 11 is a flow chart describing the algorithm executed by. of themonitor 200 for a single sender. Briefly, the algorithm (FIG. 11)proceeds as follows. Upon the arrival of each new pulse, all senderpulse sequences in the monitor's repertoire are compared with the pulsehistory contained in shift register 212. When the current pulse isdetermined to be the last pulse in a sender's pulse sequence, thatsender's code is recognized as a valid check-in and its associated timeris reset. More particularly, block 260 represents that the sequencecontrol module 220 is awaiting arrival of a next pulse from receiver204. When the next pulse arrives, the next ID interval (or pulseposition) data set is read from the repertoire file storage 224 andtransferred to the file 226. The sequence control module 220, viacontrol line 230, then sequentially supplies data representing intervalduration to register 234. Each interval duration supplied is used todetermine whether the pulse history in register 212 contains acorresponding pulse. Thus, decision block 264 asks if pulse P1 ispresent. If no, then the search for that set of ID intervals is abortedand operation loops back to examine a next set of ID intervals. On theother hand, if decision block 264 indicates that pulse P1 does exist inregister 212, then operation proceeds to decision block 266 whichexamines register 212 for the presence of pulse P2. Again, if a pulse isnot located in register 212 at the P2 position, then the search processis aborted and operation loops back from decision block 266 to block262. As is represented in FIG. 11, the pulse positions are examined insequence to determine whether the set of ID intervals loaded from therepertoire file 224 matches a pulse pattern in the register 212. If allof the ID pulses for a sender are matched by pulses in the shiftregister 212, then the associated timer 242 is reset in block 268. Afterthe search is completed, operation loops back to block 260 to await thenext pulse.

It should be apparent that the format of FIG. 9 results in each sendertransmitting a rather low density, or low duty cycle, signal. Animportant result is very low average current drain; thus long batterylife is achieved. This low pulse density reduces pulse collisionprobability and/or permits large numbers of senders to operateconcurrently with a common monitor. Transmissions from many differentsenders can be interspersed in time with no deleterious effect. Multiplepulse patterns can overlap one another in time and still be accuratelyresolved.

The effects of pulse collisions are significantly mitigated. Forexample, when two pulses collide, the system inherently interprets bothto be present. Pulses are never missed as a result of a collision. Bycontrast, in the case of serially dependent codes, any collisions thatoccur are likely to garble the received codes such that neithercolliding signal can be correctly identified. In an alternate version, anext pulse algorithm predicts the simultaneous arrival at the monitor ofpulses from two senders so that the fact of the collision isdisregarded.

In order to match received pulse sequences to the monitor's storedrepertoire, it is necessary to synchronize to the pulse sequence of eachsender. Synchronization and code detection is implemented as a singleprocess performed by the monitor of FIG. 10 as previously described.

Utilizing the frame format of FIG. 9, even if multiple sendertransmissions happen to be synchronized such that the current pulse canbe the last pulse of two or more transmitting senders, reliabledetection of all such transmitted signals can still be achieved.

The operation of searching the monitor's ID repertoire file is expeditedby aborting a search as soon as a pulse which is present in the senderrepertoire is found to be absent in the register pulse history. Inususal operation, the received pulse density is low. That means thatmost sender search events will quickly abort without the need to checkthe majority of the pulses in the sequence.

Further efficiency is achieved by allowing the sender searching andidentification processing to lag elastically with respect to thesampling operation, i.e., logging bits into shift register 212. When theprocessing requirements are high, the identification operation can fallbehind but, then can catch up a short time later. The ability to lag inthe detection process requires only that the register 212 be somewhatlonger than the actual length of the longest identification frame. Theshift register 212 may typically be implemented as a circular-buffer inRAM memory.

Some benefits of the pulse only embodiment of FIGS. 9-11 are thefollowing:

1. The communication link can be sparsely populated with pulses becauseID information is efficiently encoded by the quiet time between pulses.Thus collision probability is reduced.

2. Owing to the low density of pulses, it is possible to include largenumbers of senders simultaneously transmitting in the field of a singlemonitor.

3. Pulse collisions, when they occur, are substantially innocuous due tothe high degree of redundancy that is incorporated into the sender IDpulse patterns.

4. The sparse, short pulses of the sender output provide a low on/offduty cycle, thus low average current drain from the sender battery.

5. The use of multiple quiet intervals constitutes intrinsic redundancythat enhances detection reliability even when a sender's pulse patternhas multiple collisions with other sender units. Similarly, redundancyin the pattern improves noise tolerance.

PULSE/GROUP SYNCH (GS) EMBODIMENT (FIG. 12)

Attention is now directed to FIG. 12 which depicts the frame format of afourth embodiment (pulse/group synch). This embodiment is similar to thepulse only embodiment of FIGS. 9 in that the ID code is defined by thesequence of quiet interval durations between ID marker pulses (see FIG.12, line b). However in FIG. 12, the pulse only ID coding issupplemented by a group synch (GS) pulse pattern (FIG. 12, line a.)which also consists of a sequence of pulses which are preferablypseudorandomly spaced. The GS pulse pattern can be interspersed with thesender ID pulse pattern or the GS pattern can be transmitted separately.The GS pulse pattern is common to all senders within a given group. Agroup is defined as a population of senders sharing some property. Inmost applications, a single monitor would recognize senders of a singleGS code group.

A particular sender's pulse sequence may, for example, consist of fourGS pulses (i.e., GS1, GS2, GS3, GS4 defining GS quiet intervals R1, R2,R3) and eight ID pulses (i.e., P1-P8 defining ID quiet intervals Q1-Q7)that are interspersed as illustrated in FIG. 12, line c. Note that thelast GS pulse G4 and the eighth ID pulse P8 are preferably coincident.

FIG. 13 depicts a monitor 300 which differs from monitor 200 of FIG. 10in that it additionally includes file locations 304 for storing intervaldurations defining the positions of the GS pulses. Additionally, thesequence control module includes a control line 306 which is used tosequentially transfer the GS interval values to interval register 308.As depicted in the flow chart of FIG. 14, the detection algorithm ofmonitor 300 first searches for the completion of a set of GS pulses. Itdoes this by looking back through the long shift register 310 for acomplete GS sequence each time a new pulse is logged into register 310.In other words, the algorithm initially assumes that each new pulseentered into register 310 is the final pulse of a GS sequence and indecision blocks 320, 322, 324, it examines appropriate stages of pulsehistory register 310 to verify its assumption. Of course, most pulsesare not the final pulse of a GS sequence and so the operation can bepromptly aborted and there is no need to search the file of sender IDintervals at that pulse time.

However, if a GS sequence is detected, this indicates that a sender IDcode is present having a final pulse coincident with the final ID codepulse. So after the GS sequence is recognized in decision block 324,operation proceeds to block 326 which initiates the same ID code searchas has been discussed in connection with FIG. 11. Thus, incorporation ofthe GS pulse sequence insures that ID searching only occurs at synchtimes when a sender ID code is known to be present. This greatlyenhances processing efficiency relative to the previously describedembodiments allowing the sender repertoire to be larger. The benefits ofthe embodiment of FIGS. 12-14 over the prior embodiments include:

1. The GS synchronization mechanism simplifies and accelerates the IDdetection algorithm to permit processing larger numbers of senders.

2. The GS signal provides means for group separation so that multiplesender groups can operate simultaneously in the same area withoutburdening their associated processing algorithms.

3. The GS mechanism enhances a sender's ability to transfer informationother than identification information.

More particularly, the use of the group synchronization feature allowsthe monitor to obtain information from senders in addition to simplyidentifying them; or even, rather than identifying them. Senders can beconfigured to transmit a GS signal and also a synchronized coded pulsesequence representing a particular symbol that encapsulates information.

Each symbol has a limited set of values or identities distinguished by aparticular pulse pattern. The monitor recognizes the pattern of pulsesusing GS synchronization and decodes the symbol in much the same manneras it decodes a sender ID code. Pulse patterns of a GS signal, a symbolsignal and an ID signal can be synchronized and interspersed in themanner depicted in FIG. 12, line c.

CORRELATION DETECTION

The pulse mode embodiments of FIGS. 9 and 12 preferably incorporatecorrelation processing techniques in order to ascertain which sendersare present or which are absent. There are different modes or levels ofcorrelation processing that may be used. Consider that correlationprocessing in a most general sense involves comparing two signalpatterns. Most generally, the two patterns are both graded signals. Inthe present application, one of the signals is graded, e.g., a receivedcomposite radio baseband signal. The other signal is inherently binary,i.e., the stored ID codes.

Correlation processing generally involves sampling the signals andmultiplying the samples of the two signals and accumulating thoseproduct elements. When one signal is binary, the multiplicationoperation degenerates to addition. Specifically, a correlation value canbe obtained for each sample time by simply adding the baseband radiosample values at each point where the reference pulse pattern is unity.This correlation sum peaks when the two signal patterns have a highdegree of similarity. Peaking of the correlation sum indicates receiptof a sender pulse pattern matching that of a stored binary pulsepattern.

FIG. 15 illustrates a functional representation of such a correlationprocess. The analog baseband signal from receiver 340 is converted to agraded digital representation using an analog to digital converter 342.The digital samples produced by converter 342 are successively stored ina shifting data ram buffer 344. The correlation sum is computed bysuccessively accessing this buffer 344 at particular address locationscorresponding to the relative position of the ID and GS pulses accessedfrom files 350, 352 in the manner previous described. At each address,the value stored in the buffer 344 is summed into the correlation countaccumulator 354. A net correlation score results after all suchadditions have been completed for a particular sender ID pattern. Acorrelation score can thus be determined for each sender ID in therepertoire file 356. Similarly, a correlation score can be determinedfor the pulse pattern derived from GS file 352.

The presence of a sender is first established when a high correlationscore is obtained upon correlation of the buffer 344 with the GS pulsepattern. When the GS pattern is thus located, the sequence controlcircuit 358 proceeds to present the repertoire of sender ID patterns tothe correlation processor 354. One after another, the sender ID patternsare presented for correlation testing. A particular sender is recognizedwhen a high correlation score occurs.

FIG. 15 illustrates one possible method for discriminating correlationscores whereby a reference threshold value or discriminate is determinedin real time. In this embodiment, correlation scores are averaged overpreceding trials. This averaging function is implemented by a referencevalue averager 360. The resultant composite correlation score providedby averager 360 is multiplied by a scaling constant which is then usedas the discriminate. When a particular sender's correlation score isgreater than the discriminate, as determined by summer 364, the senderis recognized as present.

A somewhat simpler embodiment results if the radio baseband data isfirst converted to a binary stream by means of a thresholding circuit.This is illustrated in FIG. 16 where a thresholding element 370 is shownfollowing the radio receiver. Single bit data samples from thethresholding circuit 370 are stored in a long shift register or databuffer 372.

The bit address selector 374 sequentially selects particular bits fromthe shift register 372. When a true bit is selected from the shiftregister, the correlation count accumulator 376 is incremented; if afalse bit is selected, the correlation count accumulator is not changed.This process develops over each GS pulse pattern and subsequently overeach ID pulse sequence.

The correlation count accumulator output is compared to a thresholdcount, a preset value, by subtraction in summer 378. If the accumulatoroutput is greater than the threshold count value, then detection isrecognized. Before each correlation process, the correlation countaccumulator is reset by the sequence and control logic; thereupon, afresh correlation count can be determined.

One benefit of correlation processing is that it permits accuratedetection even if errors exist in the data. For example, when processingthe GS intervals, the threshold count value might be set at one or twocounts less than the possible maximum number of GS pulses. As aconsequence, synchronization can be recognized even when one or twopulses are absent from the comprised signal. Similarly, sender IDdetection can also be made tolerant of missing pulses.

GS/SYMBOL EMBODIMENT (FIG. 17)

Attention is now directed to FIG. 17 which depicts a variation of thePulse and the Pulse/GS embodiments (FIGS. 9 and 12) previouslydescribed. In the embodiment of FIG. 17, each sender's ID code consistsof multiple sub-codes. The sub-codes are here called “symbols”. Eachsymbol communicates one element from among a predefined set of elements.Each symbol might, for example, be encoded to represent a number with100 possible values ranging from 0 to 99. Three such symbols transmittedby each sender would offer the possibility of each sender'scommunicating up to 100³ or 1,000,000 possible unique identity or dataitems.

Monitor 400 operates first to recognize a GS pattern in the mannerdescribed in connection with FIGS. 12-14. Thereafter, it successivelysearches for each symbol from among the predefined set of possibleelements in the same manner as monitor 30 searched for ID codes in FIGS.12-14. For example, the monitor would search the first symbol for thecodes representing 00 then 01, then 02, etc. through 99. After findingthe code for the-first symbol, the algorithm proceeds successivelythrough the next two symbols repeating the search process for therespective symbol codes. When the monitor has completed the searchthrough all three symbols of the example, it will have obtained thethree numbers communicated by the sender and in the process will havedistinguished the code of that sender from among 1,000,000 possiblecodes. For the purpose of insight, each sender in the foregoing symbolbased system can be thought of as three senders bundled together with acommon group synch. The monitor sequentially recognizes each symbol inthe manner that sender IDs are recognized in the previously discussedpulse embodiments.

The embodiment of FIG. 17 allows the possible participation of verylarge numbers of senders in a system. As in the previously discussedembodiments, the number of senders present simultaneously in the rangeof the monitor is ultimately constrained by the overall pulse density.In general more pulses are required to transmit more symbol elements; soin most cases, the maximum number of senders that can operate in thefield of a monitor is reduced as the sender's code is elaborated withmore pulses.

Multiple symbol coding can be used to greatly expand the number ofpossible identity codes: alternatively, the symbols can be employed totransmit multiple distinct data items. Illustrating a practicalapplication of the latter, senders of a symbol based system might beaffixed to shipping cartons and used to signal the destination, inaddition to the contents or identity, of each carton as it passes down aconveyor in a freight handling facility.

The monitor 400 of FIG. 17 is depicted as monitoring three symbols. Adistinct code repertoire for each symbol is stored in storage blocks410, 412, 414. In operation the sequence and control logic 420 firstsearches for the GS pattern stored in memory 422 in the same manner asin FIG. 12. Each of the GS intervals is successively loaded into the bitaddress register 426 thereby selecting the particular sequence of bitsin the shift register for examination. If all of the GS bits are foundto be present, synchronization is recognized and a search for symbol 1intervals is initiated. The code repertoire table for symbol 1 willcontain a significant number of codes. Each code in the repertoire tableis tested for a match in the shift register. When a match is found, thesequence and control logic 422 advances to process the next symbol inlike fashion. This process proceeds through all symbols.

GS/SINGLE PULSE VARIANT

In a GS embodiment, e.g. FIG. 12, analog or digital data, as well assimple presence or absence, can be communicated to the monitor by thesender. Several examples of this capability are described in thefollowing paragraphs. The capabilities become operative and aredependent upon first achieving synchronization to a particular senderutilizing the GS and ID techniques previously described (e.g., FIG. 12).

1. Binary Status via a Single Pulse Intearated Over Time

Low bandwidth binary data can be reliably communicated by means of asingle additional pulse within the previously described format comprisedof GS and sender ID pulses. As illustrated in FIG. 18, specific pulsepositions in the sender's transmission frame are designated for datapulses. Such data pulses can then be used to indicate binary status forvarious applications such as battery low indication, window openindication, over-temperature indication, tip-over indication, valve openindication, moisture detect indication, etc.

Such slow-changing data item pulses can be detected with high confidenceover a period of several successive transmission cycles. By accumulatingor integrating the signal from one or more such specific data pulsepositions over multiple transmissions, the influence of noise orextraneous sender pulses can be effectively eliminated. For example,consider the processing of a particular data pulse from a particularsender. Each time that the monitor detects this sender it overtlysamples the additional data pulse position. The binary status of thisindicator pulse is determined after multiple successive reads of thispulse position. For example, the signal could be accumulating over eightsuccessive reads. The binary state would be determined as true when, forexample, 7 of the previous 8 reads were found to be true. The binarystate would be determined as false if less than 7 of the past 8 readswere true. This is a particularly efficient mode of data communicationbecause only a single pulse need be employed to communicate binarystatus.

Multiple binary status indicators could be transmitted by a sender byincorporating multiple data pulse positions in the waveform. Forexample, a sender having four such data pulse positions could transmitit's battery status and the status of three additional binary dataitems. The additional data items may be indigenous to the sender deviceor may be associated with electrical signals that are connected to thesender device by means of wiring.

2. Analog Data Transmitted via the Position of Multiple Pulses

FIG. 19 depicts how multiple pulse positions can be employed tocommunicate low bandwidth analog data. For example, a senderincorporating four such data pulse positions could use those four pulsepositions to encode a low resolution representation of the batterydepletion level or the environmental temperature or the flow rate in apipe. When transmitting analog data items using pulses by thistechnique, differentiating against noise and extraneous sender pulses isaccomplished by accumulating the individual pulse signals over multiplesuccessive transmissions. Having thus detected the individual pulses,those pulse positions can then be associated with binary bit positionsto allow decoding of the digital value.

3. Analog Data via the Intearated Position of a Single Pulse

In yet another example, analog data values can be encoded by theposition of a single pulse. Consider a sender of the general design aspreviously described wherein synchronization is established by the GStechnique described for sender identification. Consider that thissender's waveform includes a field of possible pulse positions forencoding analog information as indicated in FIG. 20. A continuous signalis encoded by the position of a single pulse in this field.

This embodiment presents a particularly efficient mechanism fortransmitting analog information. It is effective for low bandwidthsignals. Like the previous embodiments, this scheme relies uponaccumulating and detecting the pulse position signal over multiplesender transmissions.

Extraneous pulses due to other senders or environmental noise can beexpected. But, such extraneous pulses are removed by the integrationprocess associated with the accumulation of the pulse signal overmultiple transmissions.

4. Digital Data via Multiple “ID” Pulse Sets

In yet another example, communication of digital information at higherbandwidth is accomplished. By this technique, each sender is assignedmultiple long pulse pattern identities. This embodiment does not relyupon signal accumulation over multiple cycles. Rather, relatively fastresponse is achieved by the techniques previously described for senderidentification.

Consider, for example, a particular sender that contains in its memoryfour complete pulse pattern identity sequences. In the monitor's searchrepertoire all four of these identity sequences are associated with theone sender. The effect is as if there were four senders bundled intoone.

Binary information is conveyed to the monitor by determining whichidentity sequences are being received. In this example, four pulsepattern identity sequences provide the capability to discern two binarystate conditions. Such binary state's might indicate, for example,whether a door is open or closed and whether an infrared motion sensorhas detected a person as present.

Information of this type can change more frequently; state changes mightbe expected over the time frame of a single sender transmit cycle.

In a practical implementation, more than one of the foregoing examplescould be employed in a single device application. For example, a singlesender device might include provision to indicate its battery status bymeans of a single binary pulse and the same sender could employ multipleidentities to transmit an external binary input. That sender might alsoincorporate a pulse position field to transmit an analog temperaturemeasurement.

ACQUISITION OF REPERTOIRE

Each of the basic embodiments of this invention requires that themonitor include a repertoire of ID codes. The following are some methodsof acquiring that repertoire.

a. Pre-programmed in non-volatile memory at the factory.

b. Uploaded from a host;

c. Programmed into the unit by means of a keypad or other manualswitch-input arrangement in the field;

d. Learning sender ID codes by excluding other, more distant, inputs tothe monitor;

e. Learning sender ID codes by direct electrical contacts or by anauxiliary channel such as an IR LED;

f. Learning sender ID codes by repeated recognition.

Method a, b, and c are self-explanatory. Methods d, e, and f aredescribed in the following.

d. Learning Sender ID by Excluding Other, More Distant, Inputs to theMonitor

FIG. 21 illustrates the installation of a new sender's ID code bytemporarily limiting the receive range of the monitor. A push button 440causes the system to enter a special, acquire-new-sender, mode thatreduces the sensitivity of receiver 442 and enables the monitor torecognize and store a new sender's ID in it's repertoire.

The new sender to be installed is physically placed within thereceiver's now reduced range. Only that sender's ID is received by themonitor since all other senders are outside the reduced range. In it'snormal process, the monitor receives and tests the sender's ID code todetermine if it has been previously installed. If not, the new sender IDcode is installed in the monitor's repertoire. The acquisition processcontinues for as long as the pushbutton is depressed

FIG. 21 illustrates the acquisition process for the Pulse (FIG. 9) andthe Pulse/GS (FIG. 12) embodiments. The process is similar for the Codeburst embodiments. Cuing on the final pulse in a burst, the monitorconducts a search of its repertoire for a matching ID. If none is found,the monitor proceeds to determine the ID of the new sender as stored inthe long shift register. In that process the “SEQUENCE CONTROL” searchesfor true bits in the shift register by causing the “BIT ADDRESSSELECTOR” to increment through it's range. The interval betweensuccessive true bits is logged by the “SEQUENCE CONTROL” and thentransferred to the “INTERVAL”register. Intervals thus determined areloaded into the “FILE OF ID INTERVALS”. When all intervals aredetermined, the completed ID code is transferred to the “ID REPERTOIREFILE”.

e. Learning by Direct Electrical Contacts or by an Auxiliary Channel

FIG. 22 illustrates a system arranged to permit several methods ofacquiring new sender ID codes. As illustrated, a selector switch 450determines the method. When the selector switch is moved to the “DirectElectric” position, electric pulses from the sender coincident with itstransmitted pulses are input to the “LONG SHIFT REGISTER” 452. When the“ACQUIRE NEW SENDER” pushbutton 454 is actuated, the monitor tests thereceived ID code. If it is found to be new, it is added to the IDRepertoire File.

When the selector switch 450 is moved to the “IR SENSOR”position, thesender emits infrared ID pulses which are loaded into the “LONG SHIFTREGISTER” 452.

f. Learning Sender ID Code by Repeated Recognition

A new sender ID code can be accepted for incorporation into therepertoire if it repeatedly occurs in conjunction with a GS pulsepattern.

As previously described, a composite signal sample is repeatedly shiftedinto the “LONG SHIFT REGISTER”. With each new sample, the system firsttests for group synch (GS). The GS intervals are transferred to the “BITADDRESS SELECTOR” one at a time. With each GS bit address, the “SEQUENCECONTROL” monitors the selected bit position output of the shiftregister. As long as the bit is recognized as true (indicating a pulsereceived at that previous sample time), the sequencing continues. If onebit is missing, the process aborts on the determination that group synchis absent. When the group synch test runs to completion, that indicatesthat group synch has been detected.

When group synch is detected, the monitor first exercises the normalsender ID search process as previously described and illustrated. Then,only if a known sender in the ID Repertoire file is not recognized bythe normal search process, it may be tentatively presumed that a newunknown sender has entered the field of the monitor unit. At thatjuncture, the sequence and control logic begins testing for a new senderID pattern. A valid new sender ID pattern comprises a fixed number ofpulse detections within a defined time window. That fixed number ofpulses must occur in the clear. That is, with no other sender's pulsesinterfering.

The “SEQUENCE CONTROL” increments the “BIT ADDRESS SELECTOR” through thepossible ID pulse positions, excepting those associated with the GSpulse positions. The output of the shift register at the selectedpositions is passed to the “SEQUENCE CONTROL”which accumulates a countof “hits”.

Further, the interval between successive hits is passed to the “FILE OFID INTERVALS”. This file contains the tentatively determined ID pulseintervals. Upon completion of testing all possible ID pulse positions,the total number of hits is checked to determine if it matches anestablished threshold. If it does, the new sender is recognized.

At this juncture, the new sender ID code is regarded as tentativesubject to confirmation by detection a second time. If the same sequenceof intervals is detected again within the repetition interval of thesender, then the new sender detection is confirmed with high confidencesince the coincidence of signals from other senders could not havegenerated successive identical false ID codes within a short timeperiod. The new ID code is transferred to the “ID REPERTOIRE FILE” uponreceiving the confirming detection.

SENDER BATTERY CONSERVATION

One of the advantages of the foregoing described sender/monitorembodiments is the very low battery current drain of the senders. Byvirtue of that advantage, it is practical and advantageous toincorporate a non-replaceable battery in the senders. Further, it iscontemplated that in some applications the sender may be sealed as

protection against the elements. That could also render the batterynonreplaceable.

For reasons of test, the RF environmental protection, and unpredictabledelay between manufacturing the sender and placing it in application, itcan be beneficial to incorporate a means to turn senders ON and OFF, orat least to decrease their current drain when not in use. FIGS. 23A-23Edisclose various embodiments of the invention which accomplish that.

In FIGS. 23A and 23B, a manually operable switch is used to implementthe “on/off” function. As illustrated in FIG. 23C, the switch can beactuated through the flexible outer case of the sender. A clip typedevice as is illustrated in FIG. 23C can be used to hold the sender“off” when it is not in use. In FIG. 23A power is removed completelyfrom the sender circuitry. That may not be desirable in someapplications because of certain necessary keep-alive functions. FIG. 23Billustrates the switch removing a dc level from the micro-controller tocause it to be in a reduced power, but alive, mode.

Instead of the manually operable switch used in FIGS. 23A, 23B, amagnetic reed switch can be used which can be actuated by an externalmagnet.

FIGS. 23C and 23D illustrate an embodiment in which a phototransistor orphotodiode is arranged to sense light falling on the sender. Detectionof adequate light intensity initiates a timing sequence within thecircuitry of the sender that results in turning the sender ON eitherpermanently or for some defined time interval, for example, 24 hours.While the sender is initially enclosed in an opaque container (e.g. boxor wrapper) or, in the second instance, enclosed for longer than 24hours, it is held OFF. Either method will conserve the sender batteryfrom the time the sender is manufactured until it is first put intoservice.

For applications that are assured of operating in light during someperiod of a 24 hour cycle, the on and off control method provides ameans to shut down the sender when it is not in service. A senderattached on a person, for example, could benefit from this additionalbattery conservation feature. On or off is determined by light impingingthe sender.

As illustrated in FIG. 23D, a photo sensor is located behind a window inthe sender housing. When not in use the sender is kept in darkness.Absence of light causes the sender to switch to its reduced power mode.When placed in service, the sender is exposed to light and returns tofull normal operation.

SENDER BATTERY STATUS REPORT

In some critical applications it is necessary to know the life remainingfor the battery powering the sender. This is particularly true in thecase of senders energized by permanently installed batteries whereprecautionary replacement is not an option. Techniques for discerningand communicating battery life remaining are depicted in FIGS. 24A, 24B.

In the method illustrated in FIG. 24A each sender includes a registerthat logs the accumulated ON time and a second register that logsaccumulated OFF time. The ON time is scaled to the average power ONcurrent consumption and the measured OFF time is scaled to the lowercurrent consumption associated with sender standby current consumptionand/or battery self-discharge. The resulting battery estimate iscommunicated to the monitor as part of the sender's transmitted signal.This method can be implemented in conjunction with any of the monitorembodiments disclosed.

In a further alternative embodiment, illustrated in FIG. 24B, thebattery status report is developed from the actual battery voltage. At apredefined critical level the sender's transmitted code is modified tocommunicate its critical voltage status.

TYPICAL SPECIFICATIONS

The following are exemplary specifications that could be applicable invarious embodiments of the invention. They are not intended as limits orbounds but rather as convenient, realizable, and practical values.

RF frequency—433 Mhz

Radio range—3 to 600 feet

Active senders per monitor—2 to 1,000

Total sender population per monitor—to one million

Pulse widths—10 to 500 microseconds

Sampling resolution—one-half pulse width

Number of serial code pulses (where applicable)—3 to 30

Number of repeat cycles (where applicable)—2 to 10

Number of synch pulses (where applicable)—2 to 10

Number of ID pulses (where applicable)—3 to 30

Number of symbol pulses (where applicable)—3 to 20

Cycle repeat period—0.5 to 30 seconds

Quiet interval duration—10 to 100 pulse widths

REDUNDANCY CONSIDERATIONS

Code redundancy has a critical role in all embodiments of thisinvention. As used here redundancy is defined to be theover-determination or over-identification of code by repetition or bymultiplicity of representation.

In the serial code burst embodiment (FIG. 3), each sender repeats thecode burst at different intervals in order to enhance the prospect ofcompleting a code transmission without encountering a collision (i.e.interference from the code transmissions of other senders). The code isredundant in that multiple repetitions occur during each time outinterval. The

consequence of redundancy in the serial code burst embodiment isprimarily to reduce the probability of missed detections that mightotherwise arise due to disruption of the defined code pattern.

In the Pulse and Pulse/GS embodiments (FIGS. 9, 12) there is redundancyduring each identification frame that is conferred by the elaboration ofthe transmitted pulse pattern sequence. More pulses are employed in thecode than are minimally necessary to distinguish a given sender from allother senders. Excess pulses serve to enhance the prospect that despitea substantial fraction of pulse collisions among the signals transmittedby the population of senders, there exists a high probability that asufficient number of pulses remain in-the-clear to uniquely distinguisheach sender. The consequence of redundancy in the Pulse embodiment isprimarily to reduce the probability of false detections.

This benefit of redundancy that arises from the invention can beexamined via FIG. 9. In FIG. 9, the ID code consists of five pulses. Forconvenience in this example, let us assign the number of senders in thepopulation to be 64 and let us define the number of possible positionsfor each pulse to be 64. Each pulse position can then designate any onefrom among the 64 different senders. For example, sender A could bedesignated by position 1, sender B could be designated by position 2,etc.

Since there are five pulses in the illustrated ID frame and one pulse issufficient to uniquely designate any of the 64 senders in ourpopulation, clearly the additional four pulses are redundant. Takencollectively, the set of five pulses could have been employed todistinguish up to 64⁵ or roughly 100 million senders. The differencebetween 64 and 64⁵ illustrates the redundancy effect obtained byincorporating the additional four pulses.

Thus, redundancy serves to reduce the probability of false detections.For the pulse embodiments, at each sample, the probability of a falsedetection for a particular ID code in the repertoire, can be shown tobe:

P _(f)=(bN x)^(x)

Where P_(f) is the probability of a false detection at each sample, b isthe ratio of pulse width to repetition interval, N is the number ofsenders in the environment of the reader, and x is the number of pulsestransmitted by each sender.

In the illustrative system represented by the pulse diagram of FIG. 9,we can incorporate some practical assumptions: let's say that the pulsesare 200 microseconds in duration and the transmitted pulse patternsrepeat every 3 seconds so that:$b = {\frac{200\quad {µsec}}{3 \times 10^{6}\quad µ\quad \sec} = {6.66 \times 10^{- 5}}}$

Accordingly, for a system utilizing just two pulses, the probability ofa false detection at each sample is:

P _(f)=[(6.6×10⁻⁵)(64)(2)]²=7.3×10⁻⁵

By similar calculation if the number of pulses is increased to three,four and five then the probability of false detection, P_(f), is reducedas follows:

Two pulses P_(f)=7.3×10⁻⁵

Three pulses P_(f)=2.1×10⁻⁶

Four pulses P^(f)=8.5×10⁻⁸

Five pulses P^(f)=4.4×10⁻⁹

Thus, when this illustrative system employs an over-determined patternof five pulses rather than two pulses, the probability of a falsedetection reduces by a factor of approximately ten thousand.

PERFORMANCE ANALYSIS AND DERIVATION

Performance of systems using a pulse format, as exemplified by FIG. 9,can be characterized by the equation:

P _(f)=(bNx)^(x)

The following paragraphs provide derivation and analysis of thatequation. The analysis concludes that an optimum combination ofparameters is obtained when the density, D, of pulses is:

D=e ⁻¹=37%

The Pulse and Pulse/GS embodiments rely, in part, on statisticalbehavior to achieve accurate discrimination against false detection ofsenders. A false detection can occur under the happenstance that pulsesfrom two or more senders combine in a manner that exactly mimics thepattern of a sender that is stored in the repertoire of the monitor.Clearly, the more elaborated the pulse pattern the less likely this willhappen. Elaboration of the sender's pulse pattern is achieved byincreasing the number of pulses that comprise a sender's transmission.

The probability of mimicking a particular sender's pattern can becomputed as the combined probability that each and every pulse in thesequence will be spoofed by an extraneous sender pulse. When sender IDcodes are pseudorandomly generated, the theory of independentprobabilities applies. If the probability of falsely detecting the firstpulse is p₁ and the probability of falsely detecting the second pulse isP₂, then the probability that both are falsely detected at the samecorrelation trial is:

p _(f) =p ₁ P ₂

Clearly, the chance of a false hit on any particular pulse position isthe same as any other; that is p₁=p₂=p. So, for x number of pulses, theprobability that all pulses are falsely detected is:

P _(f) =p ^(x)

Now, the probability of falsely detecting a single pulse is equal to thefrequency or density of pulses in the environment. This overall densityof pulses, D, is due to the combined random transmissions of thepopulation of senders in the environment of the monitor. So, p=D;therefore:

P _(f) =D ^(x)

The overall density of pulses is related to the number of senders in theenvironment of the reader, N, and to the number of pulses transmittedper sender, x, and to the signal density contribution associated witheach individual pulse. The latter is defined as follows:$b = \frac{{pulse}\quad {width}}{{repetition}\quad {interval}}$

Where the pulse width refers to the pulse width of the individual pulsesand repetition interval is the time between pattern transmit cycles.

Combining those factors, the overall signal density can be expressed as:

D=bN

Substituting that into the expression for P_(f), we get the performanceequation of the system:

P _(f)=(bNx)^(x)

An alternative expression for performance can be derived for the maximumnumber of senders, N₀, which can be detected to a specified false alarmrate, P_(f0). This is obtained by solving the performance equation for Nand substituting a specified performance criterion; P_(f)=P_(f0):${N_{0}(x)} = \frac{\exp \left\lbrack \frac{\ln \left( P_{f0} \right)}{x} \right\rbrack}{bx}$

The appropriate criterion for false alarms will clearly depend upon theparticulars of the system application. For illustration, consider a30-year false alarm criterion which would be regarded as ample for mostapplications. Also for illustration, consider a system employingpractical design parameters as follow:

Pulse width=200 microseconds

Repetition interval=10 seconds

Sampling rate=10 Kilohertz

Now, b can be computed as:$b = {\frac{200(10)^{- 6}}{10} = {2(10)^{- 5}}}$

And the 30-year false alarm criterion can be related to the probabilityof false detection at each sample by the following computation:$P_{f0} = {\frac{1}{(30)(365)(24)(60)(60)(10)^{4}} = 10^{- 13}}$

Using those particular design values, the plot below is the performanceof the system, N₀(x). That is, the maximum number of senders that can belocated within the field of the monitor and still achieve the specifiedmaximum false alarm rate of 10⁻¹³ false reports per sender per sample.The abscissa of the plot is, x, the number of pulses that each sendertransmits for its unique pseudorandom pulse pattern. The plot clearlyshows that the performance of the system peaks for a particular numberof pulses, which in this case is 30. For this system, the maximum numberof senders is approximately 610; if more than 610 senders are in theenvironment of the or, then the false alarm rate is expected to exceedthe 30-year design creterion.

Clearly, different performance results will be obtained for differentvalues of the time parameter b and for different values of the falsealarm criterion P_(f0). It should also be advised that the resultsderived here represent a theoretical maximum achievable performance.Practical considerations can diminish this performance somewhat.

One practical consideration relates to the previously given assumptionthat the probability of pulse occurrences are independent. To the extentthat pulses issued by a sender are bunched together to make distinctsender transmission intervals, then the independence assumption is notentirely valid. Bunching can have the effect of increasing the falsealarm probability. Nonetheless, the theoretical performance is wellapproximated when the pulse pattern transmissions are long enough induration that the density of pulses from a single sender is much lessthan the overall density, D. This translates to a requirement that theindividual senders should transmit a pattern that is long in durationand thereby sparsely populated with pulses in order to approximate thetheoretical performance derived.

It is instructive to examine the condition of peak performance moreclosely. Returning to the performance equation, we had:

P _(f)=(bNx)^(x)

The minima of P_(f) (peak performance) occurs where the derivative ofthe performance equation is equal to zero. This derivative is of theform y=u^(v) where$\frac{u}{x} = {{{bN}\quad {and}\quad \frac{y}{x}} = 1}$

Thus,$\frac{P_{f}}{x} = {{({bNx})^{x}\left\lbrack {1 + {\ln ({bNx})}} \right\rbrack} = 0}$

Noting that the left hand term cannot be zero except in the trivialcase, we get:

1n(bNx)=−1

So,

bNx=e ⁻¹

Now, recognizing that bNx is identically the pulse density D, we get theresult that under conditions of peak performance:

D=e ⁻¹=37%

This result is general in that it applies for any set of parameters thatmay be chosen. A significant conclusion is that a system should not bedesigned to operate at a pulse density greater than 37%. Simply reducingthe number of pulses transmitted by the sender will always improve theperformance if that threshold is exceeded.

CONCLUSION

The aforedescribed embodiments of the invention can be implemented usingvarious forms of communication. Although radio signal communication maybe the most evident, it is important to note that several alternativemediums of communication can be employed. Any medium that providessenders with at least a single common channel for communication with themonitor is suitable. Some examples are: a single wire and groundconnecting all system senders to a single monitor, ultrasoundtransmitters at each sender operating with an ultrasound receiver at amonitor, and infrared transmitters at each sender which communicate withan infrared receiver at a monitor.

All of the aforedescribed embodiments can be variously implementedeither in hardware or in software or in a combination of the two. Forexample, the timing interval files can be implemented as a RAM buffer.Likewise, the long shift register can be implemented in RAM—in whichcase the bit address register may simply comprise a pointer whichindexes into the register. The ID code repertoire file may beimplemented in either RAM or ROM memory. In a software implementation,the sender timers would likely be implemented as RAM variables.Similarly, the controller can be implemented as a programmedmicrocontroller, as a state machine, or as a special purpose hardwiredlogic unit.

What is claimed is:
 1. A system for monitoring the presence or absenceof members of a defined set of members, said system comprising: aplurality of senders each capable of asynchronously transmitting auniquely encoded identification frame on a common communication channelto form a composite signal on said channel including identificationframes from said plurality of senders, each sender being uniquelyphysically associated with a different one of said members; each of saidsenders including electronic circuitry for repeatedly transmitting an auniquely encoded identification frame comprised of alternating activeand inactive intervals and where each uniquely encoded identificationframe is characterized by a unique sequence of inactive intervaldurations; said electronic circuitry including a controller forcontrolling the duration of each of said inactive intervals; and amonitor responsive to said composite signal for recognizing individualidentification frames therein for determining the presence or absence ofan identification frame unique to each sender, said monitor including:storage means for storing a repertoire of identification frames;accumulating means for accumulating a history of said composite signal;and comparator means for comparing said stored identification frames andsaid composite signal history to recognize the presence of individualidentification frames in said composite signal.
 2. The system of claim 1wherein said electronic circuitry transmits at least one pulse duringeach active interval.
 3. The system of claim 1 wherein said electroniccircuitry transmits a coded pulse burst during at least one of saidactive intervals, said coded pulse burst defining a unique senderidentification code.
 4. The system of claim 1 wherein said electroniccircuitry transmits a coded pulse burst during each of said activeintervals, said coded pulse burst defining a unique senderidentification code.
 5. The system of claim 1 wherein said electroniccircuitry of each of said senders introduces a common synchronizationpulse pattern into each of said identification frames.
 6. A system formonitoring the presence or absence of members of a defined set ofmembers, said system comprising: a plurality of senders each capable ofasynchronously transmitting a uniquely encoded identification frame on acommon communication channel to form a composite signal on said channelincluding identification frames from said plurality of senders, eachsender being uniquely physically associated with a different one of saidmembers; each of said senders including electronic circuitry forrepeatedly transmitting an identification frame comprised of alternatingactive and inactive intervals; said electronic circuitry of each of saidsenders generating a uniquely encoded identification frame comprised ofa unique sequence of inactive interval durations and including acontroller for controlling the duration of each of said inactiveintervals, a monitor for and processing said composite signal fordetermining the presence or absence of an identification frame unique toeach sender, said monitor including: storage means storing a repertoirecomprised of a different inactive interval duration sequence for eachsender; and a detector for indicating whether said composite signalincludes identification frames matching each sequence stored in saidstorage means.
 7. The system of claim 6 further including: an outputdevice responsive to said detector indicating a match for reporting thepresence of a member.
 8. The system of claim 6 further including: anoutput device responsive to said detector failing to report a match fora stored inactive interval duration sequence for reporting the absenceof a member.
 9. The system of claim 1 wherein said electronic circuitryin each of said senders includes a transmitter for generating a carriersignal and a controller for amplitude controlling said carrier signal todefine said uniquely encoded identification frame.
 10. The system ofclaim 9 wherein each transmitter in said plurality of senders generatesa common carrier signal.
 11. The system of claim 10 wherein said commoncarrier signal is a radio frequency signal.
 12. A system for detectingthe presence or absence of one or more members from a defined set ofmembers, said system comprising: a plurality of senders each capable ofasynchronously transmitting a uniquely encoded identification frame on acommon communication channel to form a composite signal on said channelincluding identification frames from said plurality of senders, eachsender configured to be uniquely physically associated with a differentone of said members; each of said senders including a power supply, atransmitter configured to be driven by said power supply to generate acommon carrier signal, and a controller for controlling said carriersignal to repeatedly transmit a uniquely encoded identification frameuniquely identifying the sender; each of said identification framescomprising a pulse identification pattern comprised of a sequence ofquiet intervals, each of said quiet intervals being bounded bysuccessive pulse intervals; and wherein the pulse identification patternproduced by each sender is characterized by a unique sequence of quietinterval durations; and a monitor for receiving said composite signalcomprising multiple pulse identification patterns transmitted by saidplurality of senders and for recognizing each different transmittedpulse identification pattern therein; repertoire means in said monitorfor storing the characteristics of a plurality of different pulseidentification patterns; and means for comparing said composite signalreceived by said monitor and the pulse identification patterns stored bysaid repertoire means to recognize the presence of individualidentification frames therein.
 13. The system of claim 12, wherein saidmeans for comparing generates a match signal when said composite signalcontains a pattern matching a stored pulse identification pattern; andan output device responsive to said match signal.
 14. The system ofclaim 13 further including: a timer defining a time-out period for eachof said senders; and means responsive to the generation of a matchsignal for resetting the timer associated with the sender identified bythat stored pulse identification pattern.
 15. The system of claim 12wherein each of said senders repeatedly transmits its unique pulseidentification pattern.
 16. A system for detecting the presence orabsence of one or more members from a defined set of members, saidsystem comprising: a plurality of senders each capable of asynchronouslytransmitting a uniquely encoded identification frame on a commoncommunication channel to form a composite signal on said channelincluding identification frames from said plurality of senders, eachsender configured to be uniquely physically associated with a differentone of said members; each of said senders including a power supply, atransmitter configured to be driven by said power supply to generate acommon carrier signal, and a controller for controlling said carriersignal to repeatedly transmit a uniquely encoded identification frameuniquely identifying the sender; each of said identification framescomprising a pulse identification pattern comprised of a sequence ofquiet intervals, each of said quiet intervals being bounded bysuccessive pulse intervals; and wherein the pulse identification patternproduced by each sender is characterized by a unique sequence of quietinterval durations; and a monitor for receiving said composite signalcomprising multiple pulse identification patterns transmitted by saidplurality of senders and for recognizing each different transmittedpulse identification pattern therein; and wherein each of said sendersrepeatedly transmits a coded pulse burst defining a unique senderidentification code during at least one pulse interval of saididentification pulse pattern.
 17. The system of claim 16 wherein each ofsaid controllers further controls said carrier signal to impose thereona pulse synchronization pattern common to all of said senders.
 18. Thesystem of claim 17 wherein the pulse identification pattern of eachsender is synchronized to said pulse synchronization pattern.
 19. Thesystem of claim 18 including means in said monitor for detecting a pulsesynchronization pattern in said received composite signal; and meansresponsive to the detection of said pulse synchronization pattern forsearching for a pulse identification pattern.
 20. The system of claim 18wherein said monitor includes means for storing a pulse synchronizationpattern and a plurality of pulse identification patterns; meansresponsive to said received composite signal containing a patternmatching said stored synchronization pattern for producing a sync matchsignal; and means responsive to said sync match signal for comparingsaid received composite signal with said stored pulse identificationpatterns.
 21. The system of claim 12 wherein the durations of said quietintervals are pseudorandomly determined.
 22. A system for detecting thepresence or absence of one or more members from a defined set ofmembers, said system comprising: a plurality of senders each capable ofasynchronously transmitting a uniquely encoded identification frame on acommon communication channel to form a composite signal on said channelincluding identification frames from said plurality of senders, eachsender configured to be uniquely physically associated with a differentone of said members; each of said senders including a power supply, atransmitter configured to be driven by said power supply to generate acommon carrier signal, and a controller for controlling said carriersignal to repeatedly transmit a uniquely encoded identification frameuniquely identifying the sender; each of said identification framescomprising a pulse identification pattern comprised of a sequence ofquiet intervals, each of said quiet intervals being bounded bysuccessive pulse intervals; and wherein the pulse identification patternproduced by each sender is characterized by a unique sequence of quietinterval durations; and a monitor for receiving said composite signalcomprising multiple pulse identification patterns transmitted by saidplurality of senders and for recognizing each different transmittedpulse identification pattern therein; and wherein the durations of saidquiet intervals fall within a range having an average durationconsiderably longer than the duration of a pulse interval.
 23. A methodfor determining whether or not each of a plurality of sender is presentwithin a detection zone, comprising the steps of: causing each of saidsenders to generate a unique identification frame comprised of activeand inactive intervals wherein the identification frame of each senderincludes at least one pulse during each active interval and a unique IDsequence comprising a unique identification pulse pattern defining aunique sequence of inactive interval durations; causing each of saidsenders to repeatedly apply its identification frame to a commoncommunication channel; allowing said Plurality of senders toasynchronously apply their respective identification frames to saidcommon communication channel to form a composite signal; causing each ofsaid senders to introduce a common synchronization pulse pattern intoeach identification frame applied to said common communication channel;monitoring said common communication channel to process said compositesignal; storing a repertoire of ID sequences, each uniquely associatedwith a different one of said defined plurality of senders; processingsaid repertoire to ascertain whether each stored ID sequence ispresented in said composite signal; determining from said processingstep whether or not each sender has applied its unique identificationframe to said common communication channel; and determining whether saidcomposite signal contains said common synchronization pulse patternprior to processing said repertoire.
 24. A method for determiningwhether or not each of a plurality of senders is present within adetection zone, comprising the steps of: causing each of said senders togenerate a unique identification frame comprised of active and inactiveintervals wherein the identification frame of each sender includes aunique ID sequence of inactive interval durations; causing each of saidsenders to repeatedly apply its identification frame to a commoncommunication channel; allowing said Plurality of senders toasynchronously apply their respective identification frames to saidcommon communication channel to form a composite signal: monitoring saidcommon communication channel to process said composite signal; storing arepertoire of ID sequences, each uniquely associated with a differentone of said defined plurality of senders; processing said repertoire toascertain whether each stored ID sequence is presented in said compositesignal; determining from said processing step whether or not each senderhas applied its unique identification frame to said common communicationchannel; maintaining a history of inactive interval durations of saidcomposite signal; and wherein said step of processing includes the stepof comparing said stored repertoire with said composite signal history.25. A system for monitoring the presence or absence of each of aplurality of senders within the detection zone of a monitor, said systemcomprising: each of said senders including a controller for generatingan identification frame including first and second pulse patternswherein said first pulse pattern is common to said plurality of sendersand said second pulse pattern is comprised of a unique sequence ofinactive interval durations uniquely associated with the generatingsender; a common communication channel; said sender controllers beingoperable to asynchronously and repeatedly apply their respectiveidentification frames to said communication channel to collectively forma composite signal on said channel; a monitor coupled to said channelfor processing said composite signal to separately identify each senderidentification frame contained in said composite signal; and whereinsaid monitor includes: a register for accumulating a history ofcomposite signal pulses; a memory for storing a repertoire of pulsepatterns; and a comparator module for comparing pulse patterns stored insaid memory with said history of composite signal pulses.
 26. A systemfor monitoring the presence or absence of each of a plurality of senderswithin the detection zone of a monitor, said system comprising: each ofsaid senders including a controller for generating an identificationframe including first and second pulse patterns wherein said first pulsepattern is common to said plurality of senders and said second pulsepattern is comprised of a unique sequence of inactive interval durationsuniquely associated with the generating sender; a common communicationchannel; said sender controllers being operable to asynchronously andrepeatedly apply their respective identification frames to saidcommunication channel to collectively form a composite signal on saidchannel; a monitor coupled to said channel for processing said compositesignal to separately identify each sender identification frame containedin said composite signal; and wherein each said first pulse pattern issynchronized relative to said second pulse pattern in each of saididentification frames.
 27. A system for monitoring the presence orabsence of each of a plurality of senders within the detection zone of amonitor, said system comprising: each of said senders including acontroller for generating an identification frame including first andsecond pulse patterns wherein said first pulse pattern is common to saidplurality of senders and said second pulse pattern is comprised of aunique sequence of inactive interval durations uniquely associated withthe generating sender; a common communication channel; said sendercontrollers being operable to asynchronously and repeatedly apply theirrespective identification frames to said communication channel tocollectively form a composite signal on said channel; a monitor coupledto said channel for processing said composite signal to separatelyidentify each sender identification frame contained in said compositesignal; said monitor including a controller for recognizing when saidcomposite signal presents said common first pulse pattern; and whereinsaid monitor controller responds to said recognition for generating acheck-in signal for each sender when said composite signal presents asecond pulse pattern associated with that sender.
 28. A system formonitoring the presence or absence of each of a plurality of senderswithin the detection zone of a monitor, said system comprising: each ofsaid senders including a controller for generating an identificationframe including first and second pulse patterns wherein said first pulsepattern is common to said plurality of senders and said second pulsepattern is comprised of a unique sequence of inactive interval durationsuniquely associated with the generating sender; a common communicationchannel; said sender controllers being operable to asynchronously andrepeatedly apply their respective identification frames to saidcommunication channel to collectively form a composite signal on saidchannel; a monitor coupled to said channel for processing said compositesignal to separately identify each sender identification frame containedin said composite signal; said monitor including a controller forgenerating a first match signal when said composite signal presents saidcommon first pulse pattern; said monitor controller including meansstoring a repertoire of sender second pulse patterns; and meansresponsive to said first match signal for comparing each of saidrepertoire of second pulse patterns to said composite signal.
 29. Thesystem of claim 28 wherein said means for comparing includes means forsequentially comparing said repertoire of second pulse patterns togenerate a sender check-in signal when a compared second pulse patternmatches a pulse pattern presented by said composite signal.
 30. A systemfor monitoring the presence or absence of each of a plurality of senderswithin the detection zone of a monitor, said system comprising: each ofsaid senders including a controller for generating an identificationframe including first and second pulse patterns wherein said first pulsepattern is common to said plurality of senders and said second pulsepattern is comprised of a unique sequence of inactive interval durationsuniquely associated with the generating sender; a common communicationchannel; said sender controllers being operable to asynchronously andrepeatedly apply their respective identification frames to saidcommunication channel to collectively form a composite signal on saidchannel; a monitor coupled to said channel for processing said compositesignal to separately identify each sender identification frame containedin said composite signal; and wherein each of said second pulse patternsis comprised of at least three pulse intervals bounding a uniquesequence of quiet interval durations in said identification frame. 31.The system of claim 30 wherein each frame first pulse pattern iscomprised of at least first and second pulse intervals fixedlypositioned in said frame; and wherein at least one of said first pulsepattern pulse intervals coincides with one of said second pulse patternpulse intervals in each of said identification frames.
 32. A system formonitoring the presence or absence of each of a plurality of senderswithin the detection zone of a monitor, said system comprising: each ofsaid senders including a controller for generating an identificationframe including first and second pulse patterns wherein said first pulsepattern is common to said plurality of senders and said second pulsepattern is comprised of a unique sequence of inactive interval durationsuniquely associated with the generating sender; a common communicationchannel; said sender controllers being operable to asynchronously andrepeatedly apply their respective identification frames to saidcommunication channel to collectively form a composite signal on saidchannel; a monitor coupled to said channel for processing said compositesignal to separately identify each sender identification frame containedin said composite signal; and wherein each of said sender second pulsepatterns is comprised of at least first and second subpatterns; saidmonitor including a controller for recognizing when said compositesignal presents said common first pulse pattern; and wherein saidmonitor controller is responsive to said recognition for separatelyidentifying said first and second subpatterns.
 33. A system formonitoring the presence or absence of each of a plurality of senderswithin the detection zone of a monitor, said system comprising: each ofsaid senders including a controller for generating an identificationframe including first and second pulse patterns wherein said first pulsepattern is common to said plurality of senders and said second pulsepattern is comprised of a unique sequence of inactive interval durationsuniquely associated with the generating sender; a common communicationchannel; said sender controllers being operable to asynchronously andrepeatedly apply their respective identification frames to saidcommunication channel to collectively form a composite signal on saidchannel; a monitor coupled to said channel for processing said compositesignal to separately identify each sender identification frame containedin said composite signal; and wherein said common first-pulse pattern iscomprised of at least two pulses fixedly positioned in eachidentification frame and spaced by a quiet interval of fixed duration;and wherein each sender second pulse pattern is comprised of at leastthree pulses in each identification frame spaced to define multiplequiet intervals, each sender second pulse pattern being comprised of aunique sequence of quiet interval durations.
 34. The system of claim 33wherein said first and second pulse patterns in each identificationframe are interleaved.
 35. The system of claim 33 wherein said firstpulse pattern and each of said second pulse patterns are synchronized ineach of said identification frames.